docs: def _get_soil_depth_term(self, C, z1pt0, vs30):
          """
          Compute and return soil depth term.  See page 1042.
          """
          # Get reference z1pt0
          z1ref = self._get_z1pt0ref(vs30)
          # Get z1pt0
          z10 = copy.deepcopy(z1pt0)
          # This is used for the calculation of the motion on reference rock
          idx = z1pt0 < 0
          z10[idx] = z1ref[idx]
          factor = np.log((z10 + 0.01) / (z1ref + 0.01))
          # Here we use a linear interpolation as suggested in the 'Application
          # guidelines' at page 1044
          # Above 700 m/s the trend is flat, but we extend the Vs30 range to
          # 6,000 m/s (basically the upper limit for mantle shear wave velocity
          # on earth) to allow extrapolation without throwing an error.
          f2 = interpolate.interp1d(
              [0.0, 150, 250, 400, 700, 1000, 6000],
              [C['a43'], C['a43'], C['a44'], C['a45'], C['a46'], C['a46'],
               C['a46']],
              kind='linear')
          return f2(vs30) * factor